1. Field of the Invention
The present invention relates generally to a Multi-Gate Field-Effect Transistor and a process thereof, and more specifically to a Multi-Gate Field-Effect Transistor and a process thereof, that forms a gradient cap layer on an epitaxial structure.
2. Description of the Prior Art
With the increasing miniaturization of semiconductor devices, various Multi-Gate Field-Effect Transistor devices have been developed. The Multi-Gate Field-Effect Transistor is advantageous for the following reasons. First, manufacturing processes of Multi-Gate Field-Effect Transistor devices can be integrated into traditional logic device processes, and thus are more compatible. In addition, since the three-dimensional structure of the Multi-Gate Field-Effect Transistor increases the overlapping area between the gate and the substrate, the channel region is controlled more effectively. This therefore reduces drain-induced barrier lowering (DIBL) effect and short channel effect. Moreover, the channel region is longer for the same gate length. Therefore, the current between the source and the drain is increased.
In order to enhance performances of Multi-Gate Field-Effect Transistors and approaching demands of size miniature, crystal strain technology has been developed and applied in the Multi-Gate Field-Effect Transistors. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strain makes Multi-Gate Field-Effect Transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel. In the known arts, attempts have been made to use a strained silicon layer, which has been grown epitaxially on a silicon substrate with an epitaxial structure disposed in between. In this type of Multi-Gate Field-Effect Transistors, a strain occurs in the epitaxial structure due to the epitaxial structure which has a larger or a lower lattice constant than the silicon one, which results in altering the band structure, and the carrier mobility is increased. This enhances the speed performance of the Multi-Gate Field-Effect Transistors.